The present invention relates to the removal of heavy metals from effluent streams. More particularly, the present invention relates to the separation of heavy metals from effluent streams by incorporating the heavy metals into buoyant floc structures that float to the surface of the effluent stream.
Numerous commercial processes require the removal of suspended solids from fluid streams prior to their recycle or emission into the environment. For example, coating operations, such as electrostatic and powder coating processes, produce effluent streams containing suspended solids.
Manufacturers commonly apply either electrostatic or powder coatings to the surfaces of metal parts, such as parts for appliances and the like. The parts must be cleaned prior to coating because manufacturers generally apply oils and lubricants to the surfaces of the parts to aid in mechanical processing. Although useful during the earlier stages of manufacture, these oils and lubricants are detrimental to any coatings which can be subsequently applied. Therefore, electrostatic and powder coating processes utilize a series of wash and rinse baths prior to coating. Further, a phosphate primer can be applied immediately prior to coating, such as a primer applied from a bath containing zinc or iron phosphate. The baths, particularly the wash and rinse baths, become contaminated over time, leading to off-quality production.
To reduce the level of contamination, a portion of each of the baths is continuously removed as an effluent stream and replaced with fresh solution. The effluent stream exiting the baths must be treated to remove the contaminants, particularly heavy metal contaminants, prior to its emission to the environment. The removal of heavy metals from industrial effluent streams is particularly problematic in light of strict governmental regulations specifying stringent levels of allowable residual heavy metals in effluent water streams emitted to the environment. Particularly stringent regulations have been established for heavy metals deemed harmful to humans, such as mercury, cadmium, zinc, copper, lead, nickel and silver. For example, regulatory agencies have introduced stringent lead standards, resulting in a demand for new treatments that are able to remove lead to extremely low levels ( less than 0.1 ppm in many cases).
Conventional wastewater treatment processes generally remove contaminants via a series of discrete chemical processes, each performed in a separate tank. For example, a conventional waste-water treatment process can consist of a series of tanks in which oils and lubricants are floated to the surface of a first tank and a number of flocculants are then added in a further series of settling tanks at different pHs to gradually precipitate the heavy metals out of the effluent. Following precipitation, the heavy metals are pumped out of the bottom of the tanks as sludge.
Many solids are slow to settle and thicken in their liquid media by gravity alone, further exacerbating the problems involved in their removal from effluent streams. Settling times of up to 3 hours are common, and it is thus frequently necessary to build relatively large settling tanks to accomplish good separation. Therefore, due to the number and size of the various tanks involved, conventional wastewater treatment processes require a significant amount of floor space. In addition, conventional wastewater treatment processes cannot process effluent streams at high flow rates. Treatment rates of about 9 gallons per minute are common, for example. In contrast, the coating processes typically supply the effluent at a much greater volumetric rate. Therefore, due to this unfortunate combination of low treatment rates and high floor space, coating processes have heretofore been forced to either shut down temporarily to allow their wastewater treatment process to catch up, coat only on a single shift but treat wastewater around the clock, or ship a portion of the effluent stream off-site for processing.
The present invention is directed to compact wastewater treatment processes capable of effectively treating effluent streams containing heavy metal contaminants at high flow rates. The instant wastewater treatment apparatus generally includes a flocculation unit, a bubble generator, a separation chamber, and means for removing the buoyant floc structures. In general, the effluent water is introduced into the flocculation unit where it is brought into contact with flocculating chemicals and air bubbles to produce buoyant floc structures. The stream exiting the flocculation unit then flows into the separation chamber, where the buoyant floc structures are subsequently removed, preferably by the use of a skimmer. In advantageous embodiments, the flocculation unit is comprised of a coiled cylindrical unit, e.g. a coiled section of process piping leading to the separation chamber. The process generally includes introducing small diameter air bubbles and a series of flocculating chemicals into the effluent stream as it flows through the flocculation unit to agglomerate the heavy metal contaminants into buoyant floc structures that float to the surface of the effluent stream. For ease of separation, the effluent stream then flows into a separation chamber at the outlet of the flocculation unit. A skimmer removes the buoyant floc from the surface of the treated effluent stream within the separation chamber.
The present invention generally provides a method of removing heavy metals from an effluent stream that includes providing an effluent stream containing heavy metals; adding an effective amount of at least one of a metal salt and a metal scavenger into the effluent stream to form floc structures including the heavy metals; adding an effective amount of a flocculating polymer to increase the size of the floc structures in the effluent stream; introducing a secondary stream containing gas bubbles into the effluent stream in an effective amount to produce an aerated effluent stream, the combination of the gas bubbles, the at least one of a metal salt and a metal scavenger, and the flocculating polymer forming buoyant floc structures that including the heavy metals; allowing the buoyant floc structures to rise to the surface of the aerated effluent stream; and removing the buoyant floc structures from the effluent stream to produce a treated effluent stream. The buoyant floc structures can be removed from the surface of the effluent stream by processes such as skimming and the like. Exemplary heavy metals which can be removed from the effluent stream include copper, nickel, zinc, lead, mercury, cadmium, silver, iron, manganese, palladium, platinum and mixtures thereof. The method of the present invention is capable of effectively treating effluent streams at flow rates of at least about 10 gallons per minute (xe2x80x9cgpmxe2x80x9d).
In advantageous embodiments, the flocculating polymer is derived from a water soluble polymer ranging in weight from about 500 to about 100,000 and further including from about 5 to about 50 mole percent of one or more functional groups capable of complexing with the heavy metals within the effluent stream to provide buoyant floc structures. In beneficial aspects of that advantageous embodiment, the water-soluble polymer is an ethylene dichloride ammonia polymer and the functional groups are dithiocarbamate salt groups.
Either or both of a metal salt and a metal scavenger are introduced into the effluent stream in various beneficial aspects of the present invention. In beneficial aspects of the present invention, both a metal salt and metal scavenger are employed. In further beneficial aspects, the metal salt is added before the metal scavenger. Exemplary metal salts include aluminum chlorohydrate, aluminum chloride, polyaluminum chloride, and mixtures thereof. In advantageous embodiments, the metal salt is aluminum chlorohydrate. The metal salt can be present in the effluent stream in an effective amount, ranging from about 25 parts per million (xe2x80x9cppmxe2x80x9d) to about 100 ppm. Exemplary metal scavengers include metal sulfides, metal (thio)carbonates, metal thiocarbamate, mercaptans and mixtures thereof. Advantageous metal sulfides include sodium polysulfide, sodium monosulfide, and sodium hydrogen sulfide. The metal scavenger can be present in the effluent stream in an effective amount ranging from about 5 ppm to about 25 ppm.
The secondary stream containing air bubbles can be supersaturated with air. The air bubbles within the aerated effluent stream preferably have diameters less than about 60 microns. The air bubbles can be formed by introducing air into a secondary stream and subsequently passing the secondary stream through a double venturi apparatus. The secondary stream containing the air bubbles can then be split into portions and introduced as separate portions into the effluent stream prior to any or all of the following steps: (i) adding the metal salt, (ii) adding the metal scavenger, (iii) adding the polymeric flocculent and (iv) removing the buoyant floc structures.
The invention further relates to an apparatus for performing the methods of the present invention. In one particularly advantageous embodiment, an apparatus for removing heavy metals from an effluent stream that includes a bubble generator for producing air bubbles in water for introduction into the effluent stream; a first chemical supply means for introducing at least one of a metal salt or metal scavenger into the effluent stream; a second chemical supply means for introducing a flocculating polymer into the effluent stream; a flocculation unit formed of a coiled cylindrical conduit having an inlet, an outlet, and a finite length which is in fluid communication with an effluent stream at its inlet and the bubble generator and the first and second chemical supply means along its length, the second chemical supply means positioned downstream of the first chemical supply means, the flocculation unit combining the at least one of a metal, salt or metal scavenger, flocculating polymer, and air bubbles with the effluent stream to form buoyant floc including the heavy metals in the effluent stream; a separation chamber in fluid communication with the outlet of the flocculation unit for separating the buoyant floc from the effluent stream by allowing the buoyant floc to rise to the surface of a volume of the effluent stream; and a means for removing the buoyant floc from the effluent stream adjacent the surface. In advantageous aspects, the first chemical supply means is positioned near the inlet of the flocculation unit and the second chemical supply means is positioned downstream of the first chemical supply means. The flocculation unit can also further include a third chemical supply means for supplying the other of the metal salt or the metal scavenger to the flocculation unit. The third chemical supply means can beneficially be located between the first chemical supply means and the second chemical supply means.
The bubble generator of the present invention can generally be formed from a conduit containing an air inlet into a water stream and two venturi orifices arranged in series. The apparatus of the present invention can further include comprising a means to divide the stream of air bubbles produced by the bubble generator into several portions, as well as a plurality of means for introducing portions of the air bubble stream into the flocculation unit at a variety of locations, such as (i) prior to the first chemical supply means; (ii) prior to the second chemical supply means; (iii) prior to the third chemical supply means and (iv) prior to the outlet of the flocculation unit.
The method of removing heavy metals according to the present invention can be used in various processes, e.g., to fabricate metal parts. Benefits of the present invention include the use of less floor space and higher throughputs than available with conventional sedimentation based wastewater treatment processes traditionally used to treat the effluent streams issuing from metal part fabrication processes. The present invention thus provides a reliable, cost effective means of removing heavy metals from effluent streams.